Method and apparatus for ultrasonic sizing of particles in suspensions

ABSTRACT

A particle size distribution monitor, comprising: a transducer adapted to be a source of ultrasonic energy and positioned in contact with a suspension containing a percent by volume of particles in a liquid, the transducer transmitting ultrasonic energy through the suspension wherein the energy comprises a wideband pulse containing a range of frequency components; a transducer adapted to be a receiver of ultrasonic energy and positioned in contact with said suspension to receive said wideband range of ultrasonic energy which has passed through the suspension; a first means adapted to accept a signal from said receiver and make an instantaneous determination of the attenuation of the wideband ultrasonic energy in passing through the suspension. A method of monitoring the particle size distribution of particles in a suspension under dynamic conditions, comprising the steps of: transmitting a first pulse of ultrasonic energy containing a wideband range of frequency components through the suspension which attenuates the pulse; receiving the attenuated pulse after it has passed through the suspension; developing a first signal representative of the attenuated first pulse; digitizing the first signal with a high speed analog-to-digital converter to form a time domain signal; applying a Fourier transform to convert the time domain signal to an equivalent frequency domain signal, or spectrum, for each signal; converting the spectrum into dB to express the attenuation as a function of frequency.

BACKGROUND OF THE INVENTION

[0001] This invention pertains to the field of particle size measurementof industrial particles and more specifically to the on-line measurementof the particle size distribution (PSD) of particles in a liquidsuspension. By suspension is meant a solid or liquid discrete particlein a liquid carrier or matrix. Examples of suspensions of interest are aslurry (a high concentration of more than about 10% to 15% solidparticles by volume in a liquid), a dispersion (a low concentration ofless than about 10% to 15% solid particles by volume in a liquid), andan emulsion (liquid particles or droplets in a liquid). It relates tothe measurement of PSD of sub-micron sized particles in a suspension,and in systems with particle sizes larger than 1 micron. It also relatesto systems used to determine the concentration and the degree ofagglomeration in multiphase systems. It relates to systems useful invarious industrial applications including emulsification (droplet size),homogenization (quality of dispersion), grinding (particle sizedistribution), precipitation of metals (particle agglomeration), andon-line measurement of particle formation (particle size distribution).

[0002] It is known that the frequency-dependent attenuation ofultrasound in suspensions is determined by the PSD within those systems(the “Forward Problem”). Several theoretical models have been developedto treat the absorption of the ultrasound for a variety of systems. Inparticular, Allegra and Hawley [Attenuation of Sound in Suspensions andEmulsions: Theory and Experiments. J. Acoust. Soc. Am. 51 (1972)1545-1564] provide a mathematical framework for calculating theattenuation of ultrasound in dispersions and emulsions. TheAllegra-Hawley model is completely general, allowing one to calculatethe absorption for new systems without having to develop new models.Although this theory is for monodisperse (single-sized) suspensions atlow volume concentrations (20% or less), it is easily extended topolydisperse systems by integrating the calculated absorption over thePSD density function.

[0003] Holmes and Challis [Ultrasonic Scattering in ConcentratedColloidal Suspensions. A. K. Holmes, R. E. Challis in Colloids andSurfaces A: Physicochemical and Engineering Aspects 77 (1993) 65-74; andA Wide Bandwidth Study of Ultrasound Velocity and Attenuation inSuspensions: Comparison of Theory with Experimental Measurements. A. K.Holmes, R. E. Challis, D. J. Wedlock in J. Colloid Interface Sci. 156(1993) 261-268] have measured absorption and phase velocity inmonodisperse polystyrene suspensions of up to 45% volume fraction andfound good agreement with the predictions of single and multiplescattering models [Multiple Scattering of Waves. P. C. Waterman, R.Truell in J. Math. Phys. 2 (1961) 512-540, and Wave Propagation Throughan Assembly of Spheres IV: Relations Between Different MultipleScattering Theories. P. Lloyd, M. V. Berry in Proc. Phys. Soc. 91 (1967)678-688]. Holmes and Challis use a wide-band pulse combined with ahigh-speed digitizer and a Fourier Transform operation to acquire theultrasonic spectrum. They use a pair of transducers at a fixedseparation for an off-line, through-transmission measurement, and ignoreall but the primary transmitted pulse. They do not demonstrate theability to measure PSD with their apparatus.

[0004] Since a known PSD can be used to predict the absorption as afunction of frequency, it should also be possible to invert ultrasonicspectra to predict the PSD based only on the absorption (“the InverseProblem”). It turns out that inverting the ultrasonic data is an art initself, as variations in this data cause instability in the standardinversion methods.

[0005] Much of the ultrasonic work to date has been concerned withmeasuring the size of relatively coarse particles (>10 microns). One ofthe first ultrasonic-based instruments was the Armco Autometrics®PSM-400 [Particle Size and Percent Solids Monitor. C. Cushman, J. Hale,V. Anderson in U.S. Pat. No. 3,779,070 (1973)], used to control grindingcircuits in the mineral industry. It used narrowband, stationarytransducer pairs (each pair operating at a single fixed frequency) and asemi-empirical model to provide an indication of median particle size(max. 600 micron).

[0006] U.S. Pat. No. 3,779,070 to Cushman et al in FIGS. 37 to 39 showsan arrangement of separate sending and receiving ultrasonic transducerson opposite sides of a ore slurry flow passage or transducers (whichboth send and receive) on one side and an ultrasound reflector on theopposite side. The transducers are in direct contact with the slurry.Slurries with mean particle diameters of from 40 to 250 microns arediscussed. Separation of the transducers is about 4.0 inches (10.2 cm).The flow passage is placed directly in a container (sump) of slurry or aslurry pipeline. One or two pairs of transducers may be used. When twotransducer pairs are used, each operates alternately, one to determineparticle size and the other to determine percent solids in essentiallythe same volume of the slurry. Two different ultrasonic frequencies maybe used depending on the attenuation expected from the sample, but oncea frequency is selected it remains constant. Selected frequencies may bewithin the range of about 0.3 to 3.0 MHz for particle size distributionswith a median size of about 150 microns or smaller. In cases where asingle pair of transducers are used (in a system using a reflector), thetwo transducers operate alternately at two different frequencies. Theultrasonic particle size measuring system provides real-time results andmay be part of a feedback loop for automatic control of a circuit forore grinding. Two major limitations are that any variations in theconcentrations must be known and the size distribution is not measuredwith this method.

[0007] Riebel and Loffler [The Fundamentals of Particle Size Analysis byUltrasonic Spectrometry. Part. Part. Syst. Charact. 6 (1989) 135-143]obtain an acoustic attenuation spectrum (2-80 MHz) with one pair ofwideband transducers to infer the entire PSD for particles rangingbetween 20 and 1000 microns. Their physical model is based on theLambert-Beer law (max. concentration 10% vol) and the assumption thatthe particle size-dependent attenuation at each applied frequency isproportional to the total particle surface encountered by the sound wavetraversing the medium. This assumption is valid only in the shortwavelength limit.

[0008] U.S. Pat. No. 4,706,509 (1987) to Riebel discloses usingultrasound for sampling multiple particle size intervals to determine aparticle size distribution preferably with 5 or more particle sizeintervals. A number of different discrete ultrasonic frequencies aresuccessively passed as tone bursts through a suspension of particles ina liquid; preferably the number of frequencies equals the number ofintervals sampled. One or more pairs of ultrasonic transmitters andreceivers may be used for through transmission measurements, or the sametransducers can serve as both a transmitter and receiver of the echoresulting from an opposed reflector. If a plurality of ultrasonic wavetransmitters are excited continuously standing waves are avoided byarranging the absorption path at an angle other than 90 degrees to thewalls of the suspension enclosure. Preferably, the frequencies selectedfor excitation are such that the wavelength corresponding to the lowestfrequency is greater than the diameter of the largest particles to beexpected, and the wavelength corresponding to the highest frequency isless than the diameter of the smallest particles to be expected. Thefrequency range contemplated by Riebel would therefore limit his methodto particles larger than 15 microns. Extensive calibration of the systemis required, and changes in particle morphology have been observed torequire a new calibration.

[0009] Recent work has tried to extend ultrasonic-based measurements tocover the sub-micron particle size regime. Pendse and Sharma [ParticleSize Distribution Analysis of Industrial Colloidal Slurries UsingUltrasonic Spectroscopy. Part. Part. Syst. Charact. 10 (1993) 229-233]report on a prototype instrument named the AcoustoPhor® (Pen Kem 8000)which comes in both off-line and on-line versions. With this system theacoustic attenuation is measured at several discrete frequencies between1 and 100 MHz. At these frequencies viscous energy dissipation of thesound wave is the dominant phenomenon for sub-micron, rigid particles.These authors Claim to be able to determine the PSD in the range of 0.01to 100 microns for slurry concentrations as high as 50% (vol).

[0010] U.S. Pat. No. 5,121,629 (1992) to Alba uses the more generalAllegra-Hawley model (which includes viscous, thermal, and scatteringcomponents) as the basis for an off-line instrument (Malvern UltraSizer)that measures PSD in the range of 0.01 to 100 microns for slurries withconcentrations up to 50% (vol). In this patent, Alba uses two widebandultrasonic transmitters and receivers working at selected, discretefrequencies within the 0.5 to 100 MHz range to determine particle sizedistribution and concentration for sizes smaller than a micron andconcentrations higher than 15% by volume. He suggests that differentembodiments could use arrangements like pulse-echo, tone bursttransmission/detection, or multiple transducers. Off-line calculatedattenuation spectra are compared to on-line measured attenuation spectraof the test sample to rapidly estimate the size distribution andconcentration via a fitting routine. He suggests the method isapplicable to both off-line and on-line operation, but it has beenobserved that the preferred embodiment of the instrument requires 4-5minutes to collect and process the data for the discrete frequencies astaught. This system is inappropriate for in-line measurements on rapidlyflowing suspensions or suspensions undergoing rapid changes.

[0011] U.S. Pat. No. 3,802,271 to Bertelson uses an acoustic signal toanalyze particles in a fluid, preferably combustion dust particles inindustrial smokestack emissions. He avoids the need to scan frequenciesby employing a complex wave shape of a non-sinusoidal waveformcomprehending several frequency components. Contemplated arerectangular, square, or sawtooth waveforms that are generated by avariable frequency oscillator that drives a speaker directly or througha pulse generator. The apparatus is said to be useful by using either afrequency scan or a single non-sinusoidal waveform. The method foranalyzing particles includes the step of generating sound wave energyhaving plural frequency components and transmitting it through the fluidwith and without the particles. A major limitation is his requirement ofa physical means to separate particles from the fluid so as to providean acoustic path without particles.

[0012] U.S. Pat. No. 5,831,150 to Sowerby et al uses a plurality ofultrasonic beams with discrete frequencies to measure particle sizes inthe sub-micron range, such as TiO₂ particles in paint samples. Solidcontent, however, was at a low percentage, such as 2.3%. One embodimentof the invention uses six pairs of piezoelectric transducers to transmitand receive ultrasound of specific frequency. Alternatively, fewerwideband ultrasonic transducers can be used to generate the tone bursts.In all of the disclosed embodiments, the PSD is estimated from theultrasonic phase velocity. A limitation is the use of a radioactivedensity gauge to measure the concentration, which is used as an input tothe ultrasonic sensor.

[0013] The above instruments, in their preferred embodiments, aresimilar in that they measure the attenuation spectrum one frequency at atime, using either swept-frequency (“chirp”) generators or a series oftone-bursts. That approach works well in the laboratory, but it iscomparatively slow at collecting data. For on-line application, thesample in the flow cell is changing as the data is collected;consequently, the upper and lower frequency components measured atdifferent times do not relate to the same moving physical particles.Other instruments that use so-called “pulse” generators actually producea pulse train, which is equivalent to a tone burst (essentially a singlefrequency or a narrow band of frequencies); such instruments mustgenerate successive tone bursts at several frequencies to measure theattenuation spectrum.

[0014] The choice of frequencies in the 1-100 MHz range taught by theprior art imposes a severe restriction on the maximum separation betweenthe transducers: for a suspension having a high concentration ofsub-micron particles (10%-50%), the attenuation is so high in that rangethat the maximum gap can be only a fraction of an inch (typically0.05-0.10 inch, 0.13-0.26 cm). Small transducer gaps are not well suitedfor on-line applications, since such small clearances tend to becomeplugged. Milling operations such as media mills and attritors aredesigned to reduce the particle size in solid/liquid slurries. Theslurries tend to be concentrated (ranging from 20-50% solids by volume),and in the case of sub-micron particles there are no in-line commercialinstruments that can measure either particle size or particle sizedistribution (PSD) of undiluted suspension without becoming pluggedafter brief operation.

[0015] In order to invert spectral data into PSD, most of the prior art(except Alba and Sowerby) uses a phenomenological model as opposed to aphysical model. Therefore the task of switching a particular instrumentfrom one process stream to another requires new calibration curves to bedeveloped.

[0016] There is a need for a method for obtaining particle sizedistribution and concentration that allows faster data acquisition at alower system cost than previous instruments. There is a need for asystem that can operate in an industrial environment and obtain on-lineresults with robust, reliable performance that requires minimalmaintenance and avoids a narrow transducer gap prone to plugging. Thereis a need for a system to invert ultrasonic spectral data into PSD basedon a physical model where switching the instrument from one processstream to another requires only substituting the appropriate physicalconstants.

[0017] In the production of precipitated particles for certainapplications, there is a need to monitor variations in an aging masterbatch of solution so that sub-batches can be withdrawn at appropriatetimes to produce particles of the correct size when combined with areducing solution that causes precipitation. U.S. Pat. No. 5,389,122 toGlicksman teaches a system for preparing finely divided, sphericalshaped silver particles (typically 1-3 microns) using a chemical agingand precipitation process. In the master, the particle size is changingduring the aging process, and any sample pulled would still be soreactive that an accurate particle size could not be obtained. There isa need for a PSD system using a simple probe that can rapidly andaccurately predict particle size and interact with an automated controlsystem to regulate the addition of a material (such as a reducing agent)to form a finely divided, spherical particle size in a stirred tanksystem.

SUMMARY OF THE INVENTION

[0018] A particle size distribution monitor, comprising: a transduceradapted to be a source of ultrasonic energy and positioned in contactwith a suspension containing a percent by volume of particles in aliquid, the transducer transmitting ultrasonic energy through thesuspension wherein the energy comprises a wideband pulse containing arange of frequency components; a transducer adapted to be a receiver ofultrasonic energy and positioned in contact with said suspension toreceive said wideband range of ultrasonic energy which has passedthrough the suspension; a first means adapted to accept a signal fromsaid receiver and make an instantaneous determination of the attenuationof the wideband ultrasonic energy in passing through the suspension.

[0019] A method of monitoring the particle size distribution ofparticles in a suspension under dynamic conditions, comprising the stepsof: transmitting a first pulse of ultrasonic energy containing awideband range of frequency components through the suspension whichattenuates the pulse; receiving the attenuated pulse after it has passedthrough the suspension; developing a first signal representative of theattenuated first pulse; digitizing the first signal with a high speedanalog-to-digital converter to form a time domain signal; applying aFourier transform to convert the time domain signal to an equivalentfrequency domain signal, or spectrum, for each signal; converting thespectrum into dB to express the attenuation as a function of frequency.

[0020] The invention is an ultrasonic PSD analyzer that uses a pulsed(time domain) technique rather than the chirp or tone-burst (frequencydomain) techniques of the prior art. The invention uses a single shortduration, wideband pulse, which contains sufficient bandwidth to obtainthe entire required frequency spectrum for the sub-micron material beinganalyzed. The single wideband pulse containing a wide spectrum offrequencies enables one to acquire the entire spectrum of interest in aninstant. A preferred embodiment of the invention includes a limitationon the frequency range to low frequencies that use longer acousticalpaths that permit large gaps between transducers in slurries having ahigh concentration of sub-micron particles.

[0021] The apparatus in one embodiment consists of a single wide-bandultrasonic transducer mounted in a flow cell, an ultrasound reflector, apulse generator, an amplifier, an analog-to-digital converter (A/D), anda computer. A wideband pulse is launched into the suspension via thetransducer, and the resulting echo train from a reflector on the farwall of the flow cell passing back through the suspension is analyzed toobtain the frequency-dependent attenuation per unit length of thesuspension. Information from multiple echoes are collected. The particlesize distribution (PSD) of the suspension can be determined from thisultrasonic spectrum through the application of an appropriate physicalmodel.

[0022] In another embodiment, an ultrasonic transducer is positionedopposite a receiver for once-through transmission of the ultrasonicsignal. The transmitter and receiver may be mounted on opposite sides ofa material passage or container, or may be mounted in a probe attachedto one side of the passage or container with the transmitter andreceiver placed a fixed distance apart. In both embodiments, the minimumgap in the acoustic pathway through which the suspension flows is atleast 0.25 inches (0.64 cm) to minimize the chance of sediment build upand clogging.

[0023] The invention is also a control method for a precipitatingparticle production system. A master batch of solution is monitoredusing the ultrasonic system of the invention and the attenuation is usedto control the withdrawal and precipitation of sub-batches in order toobtain a desired particle size distribution.

BRIEF DESCRIPTION OF THE FIGURES

[0024]FIG. 1 is a schematic of the prior art using two transducers.

[0025]FIG. 2 is a schematic of the preferred embodiment of the inventionusing one transducer.

[0026]FIGS. 3A and 3B are graphs showing the shape of a typical pulse inthe time domain (3A) and in the frequency domain (3B).

[0027]FIG. 4 is a graph showing the echo train received for a singleinput pulse.

[0028]FIG. 5A shows a general variation of attenuation and frequencyover broad ranges.

[0029]FIG. 5B is a graph showing the calculated attenuation from 10-100MHz for 2% vol. concentration of a sample of sub-micron particles inwater.

[0030]FIG. 5C is a graph showing the calculated attenuation from 100-500kHz for 20% vol concentration of the same sample as in FIG. 5B.

[0031]FIG. 6 is a graph showing a comparison between the PSD measuredwith this invention and the PSD measured via conventional means.

[0032]FIG. 7 is a side view of a reflector for ultrasonic energy.

[0033]FIG. 8 is a section view of a spool piece for a particlesuspension process line with a mounting for an ultrasonic transducer andreflector.

[0034]FIG. 9 is a section view of a probe-type assembly of a transducerand reflector in a spool piece for a particle suspension process line.

[0035]FIG. 10 is a side view of an insertable probe that contains both atransducer transmitter and a transducer receiver attached to a flangefitting on a particle suspension process line or vessel.

[0036]FIG. 11 is a block diagram of a typical control system for theparticle size analyzer.

[0037]FIG. 12 is a diagram of a silver precipitation process.

[0038]FIG. 13 is a diagram of a particle size analyzer applied to theprocess of FIG. 12.

[0039]FIG. 14 is a plot of attenuation in an aging silver solution(versus time) and the size of the corresponding precipitated silverparticles in the process of FIG. 13.

[0040]FIG. 15 is a plot of long term attenuation data illustrating theaging process in the production of silver particles.

DETAILED DESCRIPTION OF THE FIGURES

[0041]FIG. 1 shows in schematic form a typical instrument 10 common inthe prior art. The suspension to be measured is placed in a sample cell20, which includes an ultrasonic transmitting transducer 24 and anultrasonic receiving transducer 22. One of these transducers typicallymay be moved to provide a variable gap 26 between the transducers. Aprogrammable tone-burst generator 32 drives the transmitting transducer24 in a series of tone bursts at a number of predetermined frequencies.The ultrasonic pulse resulting from each tone burst propagates thoughthe suspension and reaches the receiving transducer 22, where it isconverted to an electrical signal. The amplitude of the transmittedsignal is measured by a tuned receiver 36. The signal amplitude from thetuned receiver 36 is stored by the computer 40, which initiates the nexttone burst in the sequence. At the completion of the tone-burstsequence, the computer 40 has acquired the transmission amplitude as afunction of frequency. This data, once converted to dB units,constitutes the transmission spectrum.

[0042] It should be noted that the observed transmission signal includesa contribution due to the intrinsic frequency response of thetransducer. Also there is an insertion loss at the transducer/suspensioninterface due to reflection of the ultrasound at the acousticdiscontinuity. Therefore the intrinsic instrument response must besubtracted from the measured attenuation in order to measure theattenuation of the suspension. Mechanical loading effects (which dependon the density of the suspension) can change the intrinsic response ofthe transducer. The correct determination of the intrinsic responseobviously impacts the accuracy of the attenuation measurement.

[0043] The typical prior art technique shown in FIG. 1 uses athrough-transmission method in which the spectrum is measured at twotransducer separations to correct for system element variations, oradditional transducers may be used. To obtain the attenuation of thesuspension, the difference between the “far” separation transmissionspectrum (in dB) and the “near” separation transmission spectrum isdivided by the change in the acoustic path length between the near andfar separations. This technique accounts for transducer loading andchanges in intrinsic response, but it is slow and requires a movabletransducer. When the suspensions comprise high concentrations ofparticles, at the high frequencies commonly used, the separation or gapwhere the suspension is flowing between transducers must be reduced tocounter the high attenuation if the signal strength remains the same.This presents a problem of gap clogging with the particles.

[0044] An improvement to the prior art provides the following importantcharacteristics of the invention:

[0045] (1) the use of single wideband pulses (time domain signalacquisition);

[0046] (2) the use of a large gap between transducers and the limitationof the frequency range to allow longer acoustical paths therebetween;

[0047] (3) the design of the flow cell to improve the ultrasonic signal;and

[0048] (4) the use of signal conditioning (such as logarithmicamplification) to improve the dynamic range of the instrument.

[0049] Two basic systems of practicing the invention are contemplatedfor a PSD monitor and method of operation. In a first system, thesuspension comprises a high concentration equal to 15% or greater ofsolid particles (a dense suspension or slurry) that is commonlyencountered in a milling process; preferably the concentration is 25% orgreater, but in some cases a concentration of 10% or greater may beconsidered a high concentration for purposes of this invention. Awideband ultrasonic transmitting transducer operating at a relativelylow center frequency (5 MHz or less) is mounted on one side of a passage(or flow cell installed in a pipe carrying the suspension), and awideband ultrasonic receiving transducer is mounted on an opposite sideof the passage, or alternatively a single transmitting and receivingtransducer is on one side of the passage and a reflector is mounted onthe opposite side. The flow cell may be mounted at the exit of the millfor continuous milling applications or in the recirculation pipe forbatch milling applications. In situations where the concentration is 15%or more, empirical models may be applied as is taught in the '629reference to Alba for handling concentrations up to about 70%. Forreference, it is noted that for close packed, single sized, sphericalparticles, the maximum concentration in a volume is about 50%).Preferably, the wideband pulse has a center frequency of from 100 kHz to5 MHz for determining the attenuation in a suspension comprisingparticles which make up 10% or more by volume of the suspension.

[0050] In a second system where the volume concentration is 15% or lessfor solid particles (or in the case of many emulsions, at concentrationsof up to 50%), an Allegra-Hawley model (or similar approach modified formultiple scattering) may be used to determine the PSD and particle (ordroplet) concentration. The suspension may comprise a very lowconcentration of sub-micron particles (a dispersion below about 1%particles by volume). Two wideband ultrasonic transducers spacedopposite one another for transmitting and receiving at a high centerfrequency (e.g. 25 MHz or more) are mounted on a probe that can beinserted into a process vessel (such as a stirred tank); the transducersmay also be mounted in a flow cell mounted in process piping. Thetransducers are mounted facing each other so that they areultrasonically coupled via an acoustic path through the dispersion.Alternatively, a single wideband transducer may be mounted opposite areflector for transmitting and receiving the ultrasonic pulse. Eitherthe Allegra-Hawley model or an empirical model (such as a calibrationladder created in off-line measurements of a sample) may be used toinfer the PSD and concentration, or the attenuation data itself can becorrelated directly with a process parameter of interest. Preferably,the wideband pulse has a center frequency of from 5 MHz to 50 MHz fordetermining the attenuation in a suspension comprising particles whichmake up 15% or less by volume of the suspension.

[0051] In the preferred embodiment of both systems, a widebandultrasonic pulse is generated and received by the transducers. Adigitizer (analog to digital converter) is used to capture the timedomain signal. This signal is converted into the frequency domain by aFourier transformation or one of its algorithmic implementations (suchas a fast Fourier transform FFT or digital Fourier transform DFT). Thetransmitted ultrasonic spectrum is converted into an attenuationspectrum, which is used to determine the PSD and concentration. It iscontemplated that foreknowledge of the concentration of the suspension,obtained from densitometer measurements or knowledge of the batchcomponents, would benefit the determination of PSD.

[0052] One preferred embodiment of the present invention uses a singletransducer to launch a wideband ultrasonic pulse into the suspension andto receive the resulting echo train. The term “wideband” means that thepulse contains component frequencies covering a wide range offrequencies (e.g., 200-800 kHz for concentrated suspensions, such asslurries, 15-80 MHz for dilute suspensions of submicron particles suchas in a precipitation process). FIG. 2 shows in schematic form apreferred embodiment 50 of the invention. The flow cell 55 is comprisedof a pipe section through which flows the suspension to be measured, anultrasonic transducer 62, and an ultrasound reflector 64. This reflector64 may be the far wall of the flow cell 55, but in the preferredembodiment it is an ultrasound reflector designed to maximize reflectionand minimize reverberations within the reflector itself. The reflector(64) must be designed so that only front-surface reflections arereturned; otherwise, back surface reflections will interfere with theultrasonic signal. For this reason, the opposite side of the pipegenerally is not used as a reflector. In the preferred embodiment, thereflector 64 is mounted so that its front surface is flush with theinside surface of the flow cell 55. The system clock 80 is a circuitthat provides a trigger signal to the pulser circuit 82 that drives thetransmitting transducer 62. The pulser 82 generates a single widebandelectrical pulse of short duration. The electrical pulse typically has aduration of 10 Ns and an amplitude between 5-300 volts. The transducer(62) converts this electrical pulse into a wideband ultrasonic energypulse containing a range of frequency components, which is launched intothe suspension flowing through the cell. The ultrasonic pulse propagatesthough the suspension, bounces off the reflector 64, and returns to thetransducer 62, where some of its energy is converted to an electricalsignal as the transducer now acts as a receiver. As the ultrasoundpropagates, it is attenuated according to the concentration, particlesize distribution, and composition of the suspension. The rest of theenergy continues to propagate as an ultrasonic wave from the surface ofthe transducer 62 to the reflector 64, returning to the transducer 62where more of the energy is converted to an electrical signal. Eachround trip of the ultrasonic pulse adds another echo to the receivedsignal, so that the total signal is composed of many echoes of theoriginal single pulse. This signal is amplified by a widebandpreamplifier 84 with sufficient bandwidth to preserve the spectralcontent of the echoes. The preamplifier 84 preferably includes alogarithmic amplifier (i.e. an amplifier with a logarithmic transferfunction) which compresses the dynamic range of the signal. Theamplified signal is further conditioned by a variable attenuator 85 anddigitized by a high-speed analog-to-digital (A/D) converter 86, which istriggered by the system clock 80. The digitized signal from the A/Dconverter 86 is read by the computer 90, which converts the time domainsignal into the frequency domain (ultrasonic spectra). The computer 90may provide the signal used as the system clock 80. The computer alsostores information about the intrinsic response of the fluid system(typically the attenuation of the ultrasonic signal in water) andsubtracts this from the collected signal to obtain normalized data. Thecomputer also stores characteristic data for different distributions ofparticle sizes and compares the normalized data to the stored PSD datato determine the best fit to describe the actual particle sizedistribution.

[0053] Ultrasonic propagation parameters (such as sound speed andattenuation) can vary widely in industrial processes, so the instrumentmust be able to cope with these changes. The computer controls theattenuator 85 to maximize the signal input to the converter 86 withoutexceeding the voltage range of the A/D converter, thus extending theeffective dynamic range of the converter 86. The computer 90 executes aprogram of instructions that regulates the attenuator 85, controls thedigitizer 86, and selects (i.e. “gates”) the correct portions of thedigitized signal corresponding to the echoes. The gate is typically 10microseconds or less to exclude reverberations within the transduceritself. Because temperature determines the speed with which the soundpropagates through the suspension, even minor changes in suspensiontemperature will alter the echo timing to the point where it will falloutside the gate. Therefore the computer software tracks the changes inthe position of the echo and updates the gate position to keep itcentered on the selected pulse echo.

[0054]FIG. 3A shows the time-domain pulse generated by the pulser. It isa well-known mathematical result that any time-domain signal can beresolved into its component frequencies via a Fourier transformation.The bandwidth of the resulting spectrum is inversely proportional to theduration of the corresponding time-domain pulse. Thus, the frequencycomponents of this input pulse are shown in FIG. 3B. In FIG. 3B, thefrequency has a range of from about 1 MHz to 70 MHz and a centerfrequency of about 35 MHz in the center of this range. The centerfrequency may not exactly align with the frequency having the highestsignal strength, 25 MHz, but it is usually close to it. In the practiceof this invention, it is preferred that the range of the widebandfrequency is defined by the center frequency plus and minus about 25% to70% of the center frequency value; and most preferably about 50% to 60%.This pulse travels through the flow cell and generates a number ofechoes, which form the echo train shown in FIG. 4. Using software, thedigitized signal provided by converter 86 is gated to select theappropriate echoes, then a Fourier transform (implemented as the FFTalgorithm) is used to convert the time domain signal into the equivalentfrequency domain signal (i.e., the spectrum) for each distinct echo.

[0055] The first echo in FIG. 4 travels across twice the diameter of theflow cell of FIG. 2, the second echo travels four times the diameter,and subsequent echoes travel correspondingly farther distances. Ananalysis of the returning echoes reveals that they are attenuated by thefollowing amounts:

attenuation of first echo=2I+R+2A

attenuation of second echo=2I+3R=4A

[0056] where I=insertion loss (dB), R=reflection coefficient (dB), and Ais the total absorption (dB) in the suspension for a path length equalto one diameter of the flow cell. Each of the foregoing terms is afunction of frequency. Therefore, subtracting the spectrum of the secondecho from that of the first yields a difference signal

Difference=2(R+A).

[0057] It is expected that the term R will be small compared to thequantity of interest A, and that furthermore, the term R will tend tochange slowly with time. Therefore, the difference signal represents thetotal absorption plus a small offset. Dividing this difference spectrumby the distance 2d, where “d” is the diameter of the flow cell, yieldsthe frequency dependent attenuation coefficient α(f). Thus a single echotrain provides information about the spectral content of ultrasonicpulses corresponding to a number of acoustical path lengths, withoutmoving the transducer. This technique allows one to look for anydegradation of the sensor by comparing the signals from two differentacoustic paths.

[0058] This “Multiple-Echo Spectroscopy” technique is accomplished bydetecting a first echo of the first pulse that has traveled over a firstpath length and determining the spectrum of the first echo; detecting asecond echo of the first pulse that has traveled over a second pathlength and determining the spectrum of the second echo; determining thedifference between the first echo spectrum and second echo spectrum anddividing the difference by the difference between the first path lengthand second path length to obtain an attenuation independent of systemvariations.

[0059] Also, the system of a reflector and a combination transmitter andreceiver, as in FIG. 2, compared to no reflector and a separatetransmitter and receiver (one of which would have to move to collectdegradation information), as in FIG. 1, is a lower cost system forcollecting the required ultrasonic spectra.

[0060] The technique just described depends upon the total attenuationbeing sufficiently low to allow an ultrasonic pulse to propagate acrossa distance equal to several diameters of the flow cell. The prior artstrongly favors the choice of high frequencies (above about 50 MHz),where the attenuation is very strong for high concentration suspensions,or slurries, of sub-micron particles. FIG. 5A shows a qualitativedescription of attenuation coefficient (dB/in), or just attenuation, forseveral particle sizes (0.20-0.28 m) over a wide range of frequencieswhere the low frequency regime 87 is shown in an expanded scale comparedto the high frequency regime 89.

[0061]FIG. 5B shows the attenuation for several particle sizes(0.20-0.28 μm) in the high frequency regime 89 at a concentration ofabout 2% as calculated with the Allegra-Hawley model, which has beenconfirmed by empirical results. It is evident that even at lowconcentrations (2% concentration as shown in FIG. 5B), the attenuationloss is so high (250-450 dB/in) that the gap 26 between transducersshown in FIG. 1 can be at most a small fraction of an inch (less than2.54 cm) for practical ultrasonic energy levels. Such a gap would beinappropriate for on-line applications where the suspension wouldquickly clog the gap. In addition, multiple echoes could not bedistinguished in a system with such a small gap. It should be noted inFIG. 5B that the differentiation between different particle sizesbecomes more distinct as the frequency is increased. Thus, the naturalinclination, as evidenced by the prior art, is to move towards higherfrequencies. This choice of frequency regime is incompatible with thepresent invention having a large gap, of preferably at least about 0.25inches (0.64 cm), when there is a high concentration of particles, andespecially sub-micron particles.

[0062] In the case of a high concentration of particles, particularlysub-micron particles, a better choice of a frequency range is below 50MHz and preferably below 1 MHz. FIG. 5C shows the attenuation (dB/in)calculated in a 100-500 kHz range at a concentration of about 20%. Inspite of the higher concentration (20% shown in FIG. 5C), theattenuation is two orders of magnitude smaller. Thus the propagationdistance can be long enough to allow Multiple Echo Spectroscopy to beused. At 500 kHz, the attenuation curves in FIG. 5C show as muchrelative differentiation as they do at 100 MHz in FIG. 5B. Therefore noloss of sensitivity using a large gap (of at least 0.25 inches, 0.64 cm)is expected in this frequency regime.

[0063] The prior art teaches that by determining the absorptioncoefficient α_(ij) at many frequencies f_(j) (using the Allegra-Hawleymodel for example) and the absorption spectrum A_(j) of an unknownsuspension, it is possible to determine the PSD in terms of theconcentration of particles c_(i) at each size i simply by solving (i.e.,inverting) the linear equation below:$A_{j} = {\sum\limits_{i = 1}^{n}\quad {\alpha_{i\quad j}c_{i}}}$

[0064] Past experience shows there is so little information in theattenuation spectra for sub-micron particles that inversion of thelinear equation is very unstable and yields only 2 or 3 non-zero sizeclasses. It has been found that the only robust method of extractingparticle size information out of measured spectra is to parameterize thePSD. A simple example is the assumption of a log-normal distribution forthe PSD function. By integrating the Allegra-Hawley attenuation over alog-normal distribution, the number of free parameters is reduced tothree: concentration, mean size, and distribution width. Thesequantities can be extracted by fitting the parameterized model to theultrasonic spectrum. An example of the results of this approach isdepicted in FIG. 6. For a given sample, the results from the monitor ofthe invention are indicated by the solid line 97 showing the cumulativesize distribution of the particles. The data curve depicted withcircles, such as circle 99, indicate the results of the same sampleanalyzed with a conventional Brookhaven x-ray disc centrifuge. Theresults obtained with the monitor compare favorably with the results ofthe Brookhaven instrument.

[0065] The reflector must be designed to suppress reverberations thatcause extraneous echoes to be received by the transducer. A simple metalplate (or the opposite wall of the pipe) will generally give a confusingecho train that is difficult to interpret. For example, the powerreflection coefficient at a water/steel interface is about 88%.Therefore about 12% of the ultrasonic energy enters the steel; at thefar wall of the reflector most (88%) of this energy is reflected. Aftera reverberation time equal to twice the reflector thickness divided bythe speed of sound in steel, about 12% of this internal energy emergesas a back surface reflection. The acoustic energy trapped in thereflector continues to be emitted at constant intervals (equal to thereverberation time) until the energy is dissipated. Depending on thematerial and geometry of the reflector and the transducer-reflectorspacing, these back surface echoes will arrive at the transducer atabout the same time as the desired front surface reflection; thespurious signal will have an intensity about 18 dB below that of thedesired signal, which is sufficiently strong to distort the ultrasonicwave. The Fourier transform of the distorted wave will yield a spectrumthat does not accurately capture the frequency dependent attenuation ofthe suspension itself as has been observed.

[0066] There are many methods to reduce the reverberations produced bythe reflector. One might be to place impedance matching layers or anacoustic absorber on the back surface. An alternative approach is shownin FIG. 7, but it should be understood that this example is not the onlysolution to the problem of reverberation. FIG. 7 shows a side view of areflector made from 316 stainless steel round bar stock. The diameter100 is approximately 1.5 inches (3.81 cm); one end has been milled awayat a 30° angle, and both ends have been sanded to a smooth finish. Theultrasonic beam is incident at right angles to the reflector on theright side of the figure; 88% of the sound is reflected and 12% entersthe reflector. Upon reaching the back surface of the reflector, theinternal sound beam is reflected at an angle due to the inclination ofthe surface. When this secondary beam reaches the front surface, it isincident on the front interface at a 60° angle. Due to Snell's Law, thissound beam is refracted and emerges into the suspension at about 10°from the normal. The central part of this delayed and refracted beammisses the transducer, which is about 2 inches (5.1 cm) from thereflector. Also, the main lobe of the transducer's sensitivity pattern(assuming 500 kHz and a transducer diameter of 1 inch (2.54 cm)) onlyextends to 8.4°; therefore, the only echoes seen by the transducer arethose due to front surface reflections.

[0067] Without proper alignment of the transducer with respect to thereflector, phase cancellation effects will distort the received signalat high frequencies (above 20 MHz). The transducer 102 shown in FIG. 8is mounted in a bore 104 in a spherical bearing 106 (which provides bothgimbal and swivel). The transducer acts as a transmitter for the pulseof ultrasonic energy and thereafter acts a receiver for the reflectedsignal. The bearing 106 is sealed to the flow cell with an o-ring 108.This o-ring provides some measure of acoustic and electrical isolationbetween the transducer and the rest of the system. Since mechanicalsources of acoustical noise tend to be in the low frequency range, themain concern here is with shear waves (produced by mode conversion)coupling into the transducer. The transducer 102 and its mounting arepart of a spool piece 110 that would bolt into an existing pipeline andwould become an integral part of the equipment. A reflector 112, asdiscussed referring to FIG. 7, would be placed in the spool pieceopposite the transducer.

[0068] An alternative to the arrangement of FIG. 8 is an insertableprobe 114 shown in FIG. 9. The probe would contain both transducer 102and reflector 112, and it could be inserted through the bore of aball-valve 116 into an existing process stream. The attraction of theprobe is that is can be inserted or removed while the process isrunning. The transducer acts as both a transmitter and receiver as inthe embodiment of FIG. 8.

[0069] A further alternative arrangement is shown in FIG. 10 in which aninsertable probe 118 contains both an ultrasonic transducer transmitter120 and an ultrasonic transducer receiver 122. The probe is attached toa flange fitting 124 in a process vessel or pipe 126 shown in dashedlines. The transmitter 120 and receiver 122 are separated by a gap orsensing volume 128 that is large enough not to become clogged with thesuspension being analyzed. The received ultrasonic signal is amplifiedand conveyed via a 200 MHz bandwidth fiber optic link 132 to theattenuator and digitizer residing in the computer 130. This is also thepreferred way to connect a computer for the arrangements of FIGS. 8 and9.

[0070] In order to convert the measurements of frequency-dependentattenuation into particle size distribution (PSD), a physical model isneeded to give the relation between attenuation and particle size. It isuseful to employ the Allegra-Hawley model because it is valid for bothdispersions and emulsions. This model is used to calculate the expectedattenuation as a function of particle diameter and frequency. Theresulting matrix of pre-calculated values is stored in the computer.When measurement is made, a fitting routine is used,to determine thebest estimates of the log-normal parameters (median size, spread, andconcentration) that define a PSD over which the Allegra-Hawley matrix isintegrated, so that the resulting predicted attenuation agrees with theobserved attenuation. A simplified flow chart is shown in FIG. 11 is oneexample of a way of operating the system of FIG. 2. It involves use ofthe Allegra-Hawley model and as such is limited to dilute aqueoussuspensions of less than about 15% by volume solids concentration andless than about 50% by volume liquid in liquid concentration foremulsions. For higher concentrations one solution is to handle thesuspension with an automated sampling system that dilutes the suspensionto appropriate levels to use Allegra-Hawley. The Allegra-Hawley matrixis first calculated off-line for a range of particle size distributionsand the results are stored for later use on the computer 90. Theultrasonic transducer 62 is triggered by the pulser 82 and the reflectedsignals are collected, preamplified with amplifier 84 having thelogarithmic stage 85, and digitized with a high speed A/D converter 86.The digitized signals are analyzed to locate the first reflection (A)and second reflection (B) which are converted to transmission spectra(dB vs. frequency). The attenuation is determined by the formulaattenuation=(A−B)/distance, where the distance is the path length of thesignal from transmitter to receiver, which in the case of the reflectorsystem in FIG. 2, would be the distance from the transducer face (actingas a transmitter) to the reflector face and back to the transducer face(acting as a receiver). This treatment eliminates the effect ofundesired transducer output drift. To increase the signal to noiseratio, this cycle would be repeated about 50 times and the 50attenuation spectra averaged. The measured average signal would becompared to the expected attenuation (based on an estimate of the PSDand the stored Allegra-Hawley matrix) to determine if there is a goodfit. If the fit is not good enough, an adjustment is made to theestimated PSD and the attenuation is compared again until a good fit isobtained. The particle size distribution determined is displayed as anoutput showing median size, spread, and concentration. The measuredattenuation is plotted against the best fit estimated attenuation.

[0071] The flow chart of FIG. 11 can be modified to accommodate thetransmitter/receiver arrangement of FIG. 10 or something similar to theprior art arrangement of FIG. 1 (through transmission systems) byeliminating the steps for looking for reflections. Transducer driftcould be corrected for by some other means, such as moving onetransducer and comparing the signals from the two different pathlengths. For transducers where stability and rapid buildup of particleson the transducer is not a concern, routine corrections for drift maynot be necessary. The flow chart can also be modified to apply toslurries with particle concentrations greater than 20% by substitutingan empirical model for the Allegra-Hawley calculation. Such empiricalestimating steps are found in the '629 reference to Alba. The system ofthe invention is especially useful where the particle distributionincludes sub-micron particles and the suspension has more than 20% byvolume of particles. In this case, a low frequency ultrasonic widebandsignal would be employed.

[0072] One application of the particle size measuring system is in amilling process where a material, such as CaCO₃ is being broken downinto fine particles for an intended use. In the case of the millingmonitor, referring to FIGS. 2 and 11, the steps involve:

[0073] making a flow cell with a two inch (5.1 cm) inner diameter and awideband transducer mounted opposite an ultrasonic reflector so that thephysical gap between them is two inches (5.1 cm) (giving an acousticalpath length of four inches, 10.2 cm);

[0074] generating a wide bandwidth ultrasonic pulse at a centerfrequency of about 500 kHz and a frequency range spanning 200 kHz to 800kHz and injecting it into the suspension;

[0075] receiving an attenuated wide bandwidth ultrasonic pulse at acenter frequency of about 500 kHz and a frequency range spanning 200 kHzto 800 kHz that has passed through the suspension;

[0076] preamplifying the received attenuated signal with a logarithmicamplifier stage;

[0077] digitizing the preamplified signal with a digitizer with a 10 MHzsampling rate;

[0078] gating the digitized signal to eliminate “ringing” developedwithin the transducer faceplate;

[0079] applying a Fourier transformation (e.g. FFT) to the signal;

[0080] subtracting the baseline signal for water to get a preprocessedsignal;

[0081] dividing the preprocessed signal by the acoustical pathlength todetermine a normalized attenuation (dB/in);

[0082] assuming a log normal size distribution;

[0083] calculating an assumed PSD using estimated log-normal parameters(median size, distribution width, concentration);

[0084] using the Allegra-Hawley model (or other theoretical or empiricalmodel) to calculate the expected ultrasonic attenuation based on thecalculated PSD;

[0085] comparing the fit of the expected ultrasonic attenuation to theobserved normalized attenuation. Iterate by adjusting the estimate ofthe log-normal parameters until a best fit is obtained;

[0086] estimating the PSD based on the final estimate of the log-normalparameters.

[0087] A similar application involved milling of TiO₂ using twotransducers (through transmission mode) mounted in a two inch (5.1 cm)diameter flow cell (giving a two inch (5.1 cm) gap). It is contemplatedthat in the absence of a reliable theoretical model (such as may be thecase at very high concentrations), this method may be adapted toestimate the relative change in PSD by comparing the normalizedattenuation signal to empirical attenuation data corresponding to avariety of particle size distributions for that process material. It isfurther contemplated that this method can be applied to other PSD modelsbesides the log-normal model.

[0088] Another application of the particle size measuring system is fora dilute suspension, or dispersion, where the particle size is changingand where the particles are sub-micron and are 15% or less by volume. Inthis case, a high frequency wideband ultrasonic signal would beemployed. One such application is described in FIG. 12 showing a silverprecipitation system. The system comprises a master batch solution tank150, a sub-batch weigh tank 151, a second solution tank 154, a sub-batchprecipitation tank 152, a filter and wash device 153, an off-line sizemeasurement device 155, and a particle-to-paste processing line 156. Amaster batch of silver solution is prepared from a metal salt solution,such as silver nitrate, to which is added MEA (monoethanolamine) to forma solution A at 150 in which aging and growth of particles occurs. It isdesired to remove solution A in sub-batches to precipitate out thesilver when the aging process has reached at point at which particles ofthe desired size can be produced. The sub-batch is weighed out andcollected in tank 152 and a given quantity of reducing agent, solution Bat 154, is added to precipitate silver particles which are thenfiltered, washed, and freeze dried to be used later in the process toprovide a desired end product, such as a paste. Additional details ofthis process can be found in the '122 reference to Glicksman,incorporated herein by reference. In the past, it was not accuratelyknown when to withdraw the sub-batch to achieve the desired silverparticle size. In the past, it was thought that a period of aging ofabout 16 hours was required. It was also not known how the master batchparticle size continued to vary over the time multiple sub-batches werewithdrawn. The master batch aging particles are changing in size earlyin the process and any sample pulled would still be so reactive thataccurate particle size could not be obtained from a withdrawn sample. Byapplying the particle size sensing system of the invention at the masterbatch tank 150, a relationship was discovered between the attenuationsignal of the aging particles and the final silver particle size.

[0089]FIG. 13 shows the probe 118 of FIG. 10 located near the bottom ofstirred tank 150. An experiment was run to study changes in theultrasonic attenuation due to aging of solution A in tank 150 and relatethat to desired silver particle size obtained from tank 152. Theattenuation was measured continuously starting with the addition of MEAto the silver nitrate solution. A background reading was taken usingdeionized water. The probe was operated at 50 MHz with a transducer gap128 (FIG. 10) of about ¾ inch (1.9 cm). The probe is also known to workat 30 MHz with a transducer gap of about 1¾ inches (4.4 cm).Periodically, a sample was drawn from tank 150 and combined with a fixedconcentration of solution B to produce silver particles byprecipitation. These silver particles were later imaged via SEM and theresulting images were analyzed to determine silver particle size. Theimages were analyzed using NIH Image software to determine the averageand standard deviation of the particle diameter for 20 particles.

[0090] The attenuation and size data are compared in FIG. 14. It isevident that the attenuation of the aging particles in solution A (shownas small dots) rises and corresponds with an increasing particle size ofthe precipitated silver particles (shown as a filled circle representingthe average particle size and a line representing the particle sizerange of the 20 particles in the sample). The attenuation scale is onthe left and the particle size scale is on the right of the plot. Theaging time (i.e., when the sample was drawn and precipitated) is shownon the horizontal axis. The attenuation and particle size increase withaging time until a plateau is reached in particle size and theattenuation decreases at about 180 minutes of aging.

[0091] The test indicates the particle size analysis system is capableof monitoring the aging process in solution A, and there is apredictable relationship between the attenuation and the final silverparticle size. It is known that the particle size can be adjusted byvarying the composition of solution B. In the past this adjustment couldonly be done by trial and error since the particle size was not knownuntil one sub-batch was treated with solution B and the particle sizemeasured by device 155. Then adjustment could be made on the nextsub-batch. This adjustment may be off, though, since the aging processchanges the particle size from one sub-batch to the next. Sometimessub-batches would have to be blended to compensate for undesirableparticle size variation, adding cost and time to the production process.By knowing the variation in aging particle size using the monitor, thecomposition of solution B can be determined with greater certainty sothe particle size of the silver particles can be maintained relativelyconstant regardless of when the sub-batch is withdrawn from the masterbatch.

[0092] These measurements of solution A can be made in-line in real-timeat a silver particle plant. FIG. 15 illustrates that the initial agingprocess has completed after only about 3 hours, so a campaign ofmultiple sub-batches can be started after 3 hours instead of after the16 hours that was previously considered necessary. This discoveryprovides a basis for a shortened aging cycle that could increaseproductivity at the plant by 20%. As the master batch ages and theattenuation slowly decreases as seen in FIG. 15, the PSD monitor can beused to regulate the concentration of solution B to add to a particularsub-batch to get a predictable, uniform, silver particle size from thedifferent sub-batches. Each master batch may also be slightly differentfrom other master batches so compensation can be made from master batchto master batch by monitoring the aging particle size. It is believedthe invention can be usefully applied to other precipitation processesfor precipitating particles from a solution, such as processes forprecipitating a metal particle from a metal salt solution to obtainparticles of gold, paladium, platinum and the like.

What is claimed is:
 1. A particle size distribution monitor, comprising:a transducer adapted to be a source of ultrasonic energy and positionedin contact with a suspension containing a percent by volume of particlesin a liquid, the transducer transmitting ultrasonic energy through thesuspension wherein the energy comprises a wideband pulse containing arange of frequency components; a transducer adapted to be a receiver ofultrasonic energy and positioned in contact with said suspension toreceive said wideband range of ultrasonic energy which has passedthrough the suspension; a first means adapted to accept a signal fromsaid receiver and make an instantaneous determination of the attenuationof the wideband ultrasonic energy in passing through the suspension. 2.The monitor of claim 1, further comprising a second means adapted todevelop an output representative of the total particle size distributionof the suspension.
 3. The monitor of claim 1, wherein the first meansincludes a logarithmic preamplifier for said signal.
 4. The monitor ofclaim 1, wherein the first means comprises an A/D digitizer for saidsignal, an FFT analyzer for the digitized signal, and means to obtainthe magnitude of the FFT data representing the measured attenuation; andthe second means comprises means to determine an estimated PSD and meansto compare the estimated PSD with the measured attenuation and determinethe goodness of the fit.
 5. The monitor of claim 4, wherein the firstmeans further comprises a logarithmic preamplifier that compresses saidsignal from the receiver before it is processed by the A/D digitizer. 6.The monitor of claim 4, wherein the first means further comprises ameans of controlling an additional attenuation of said signal from thereceiver before it is processed by the A/D digitizer.
 7. The monitor ofclaim 1, wherein the source of ultrasonic energy has a center frequencyin the range of from 100 kHz to 5 MHz for determining the attenuation ina suspension comprising particles which make up 10% or more by volume ofthe suspension.
 8. The monitor of claim 1, wherein the source ofultrasonic energy has a center frequency in the range of from 5 MHz to50 MHz for determining the attenuation in a suspension comprisingparticles which make up 15% or less by volume of the suspension.
 9. Themonitor of claim 1, wherein the transducer to transmit the beam ofenergy and the transducer to receive the beam of energy are the sametransducer and further comprising a reflector positioned in thesuspension opposite the transducer and wherein the transducer is adaptedto receive multiple echoes of energy from the reflector and determine acorrected attenuation value from the information in the multiple echoes.10. The monitor of claim 1, wherein the first means adapted to make aninstantaneous determination of the attenuation of the ultrasonic energyin passing through the suspension is a computer that is remotelypositioned from the transducers and is connected to the transducers by afiber optic cable.
 11. A method of monitoring the particle sizedistribution of particles in a suspension under dynamic conditions,comprising the steps of: transmitting a first pulse of ultrasonic energycontaining a wideband range of frequency components through thesuspension which attenuates the pulse; receiving the attenuated pulseafter it has passed through the suspension; developing a first signalrepresentative of the attenuated first pulse; digitizing the firstsignal with a high speed analog-to-digital converter to form a timedomain signal; applying a Fourier transform to convert the time domainsignal to an equivalent frequency domain signal, or spectrum, for eachsignal; converting the spectrum into dB to express the attenuation as afunction of frequency.
 12. The method of claim 11, further comprisingdeveloping the attenuation to determine the total particle sizedistribution of the suspension and presenting said determination as anoutput.
 13. The method of claim 11, wherein the step of transmitting afirst pulse of ultrasonic energy containing a wideband range offrequency components through the suspension, comprises transmitting theenergy through a suspension comprising sub-micron particles.
 14. Themethod of claim 11, further comprising: reflecting the first pulse witha reflector; detecting a first echo of the first pulse that has traveledover a first path length and determining the spectrum of the first echo;detecting a second echo of the first pulse that has traveled over asecond path length and determining the spectrum of the second echo;determining the difference between the first echo spectrum and secondecho spectrum and dividing the difference by the difference between thefirst path length and second path length to obtain an attenuationcorrected for system variations.
 15. The method of claim 11, wherein thestep of transmitting a first pulse of ultrasonic energy containing awideband range of frequency components through the suspension comprisestransmitting through a suspension containing 10% or more of particles byvolume and the wide bandwidth of the first pulse has a center frequencyof 5 MHz or less.
 16. The method of claim 11, wherein the step oftransmitting a first pulse of ultrasonic energy containing a widebandrange of frequency components through the suspension comprisestransmitting through a suspension containing 10% or less of particles byvolume and the wide bandwidth of the first pulse has a center frequencyof 5 MHz or more.
 17. The method of claim 11, wherein the transmittingand receiving occur along an acoustic path that has a length greaterthan 0.25 inches (0.64 cm).
 18. A method of utilizing particle sizemonitoring to control a process for precipitating particles, comprising:repeatedly applying the method of claim 11 to collect data continuouslyabout the attenuation of an ultrasonic pulse passed through a masterbatch of a solution which has a particle size that is changing;generating a signal when the particle size reaches a predeterminedattenuation value; withdrawing a sub-batch of solution after the signalis generated; predetermining a concentration of a second solution to beadded to the withdrawn sub-batch based on the measured attenuationsignal at the time of withdrawal; adding said predeterminedconcentration of second solution to the sub-batch to precipitateparticles.
 19. The method of claim 18, wherein the master batch is ametal salt solution and the particles precipitated are metal particles.20. The method of claim 19, wherein the metal salt solution is a silversalt solution and the particles precipitated are silver particles.